To learn more, see:

Yucha, C. & Gilbert, C. (2004) Evidence-Based Practice in Biofeedback and Neurofeedback. Association for Applied Psychophysiology and Biofeedback. Colorado Springs, CO.

The strict criteria for each rating level are further explained in the Journal of Neurotherapy, 6, 11-23. 

Also see:

Moss, D. & Gunkelman, J. (2002). Task force report on methodology and empirically supported treatments: Introduction. Applied Psychophysiology and Biofeedback, 27, 261-262.